Leaders for warring sides in South Sudan said today they are close to signing a cease-fire in a monthlong conflict that the UN says has created a "horrifying human rights disaster."

Military spokesman Philip Agur, meanwhile, said army forces retook the key city of Bor today, defeating a coalition of some 15,000 rebels.

There are doubts about how much control rebel leader and former Vice President Riek Machar has over some of the forces fighting for him, raising concerns about his ability to reign in all the rebels even if he signs an agreement.

In Juba, the capital of South Sudan, presidential spokesman Ateny Wek Ateny said he believed a cease-fire would be signed in the next two days.

In Ethiopia, where talks are taking place, rebel negotiator Mabior de Garang said he hoped the agreement would be signed later today. The rebels apparently have made a major concession, according to negotiators, agreeing the cease-fire deal will not be tied to previous demands for the immediate release of 11 key political detainees.

That sticking point and other issues, including the withdrawal of Ugandan troops fighting for the government, are to be discussed after a cease-fire is signed and hostilities end, they indicated.

De Garang is the son of former rebel leader John Garang, who prosecuted the nearly 40-year rebellion against Sudan only to die months after negotiating a 2005 peace agreement that laid the way for South Sudan's separation from the mainly Arab-led north.

The latest conflict threatened to splinter along tribal lines the country that became Africa's newest nation in July 2011.

Fighting broke out December 15 and spread throughout the country after whole army units defected. Thousands are believed dead and hundreds of thousands have been forced from their homes.

The top UN human rights official, Ivan Simonovic, yesterday charged both sides have committed mass atrocities, with mass killings, gruesome atrocities, sexual violence and using children to fight.

He called it a "horrifying human rights disaster."

The United Nations also has accused both sides of looting from humanitarian organisations.

Agur, the military spokesman, today denied troops were involved in stealing and human rights violations, even as he urged soldiers to behave lawfully.